A nuclear reactor power installation conventionally comprises a reactor building providing a pressure-proof containment for the reactor. One or more steam generators are connected to the reactor by core coolant pipes providing the primary medium for each steam generator. The steam generator contains a heat exchanger through which the reactor coolant flows internally, and which is contained within a housing fed with feed water with the housing having a live-steam outlet. The outlet is provided with a quick-acting shut-off valve through which a live-steam pipe is connected to the generator's steam outlet, the pipe extending from the generator to and through the containment and onto its point of useful consumption, such as a steam turbine. All of this applies to each steam generator involved by the installation. In each instance, the shut-off valve provides an internal flow passage having a cross-sectional area corresponding to that of the live-steam pipe, and possibly a larger area, to avoid the valve retarding the steam flow appreciably.
The purpose of such a fast-acting valve, particularly in the case of a pressurized-water reactor steam generator having a multiplicity of thin-walled heat-exchanger tubes forming a barrier or boundary between the reactive core coolant and the steam, is to avoid loss of steam via a broken live-steam pipe, at a velocity capable of damaging the heat-exchanger tubes.
At the same time, when the quick-acting shut-off valve closes, the steam pressure in the steam generator can rise to dangerous values even if the reactor is shut down as quickly as possible.
Consequently, a problem is involved because there is the conflict between the need for the quick-acting shut-off valve to as immediately as possible stop the possible rush of steam from the steam generator at damaging velocity which at the same time presents the hazard of an excessive steam pressure rise within the steam generator.